I think this is an appropriate solution:
<select id="test" name="form_select" onchange="showDiv(this)">
<option value="0">No</option>
<option value="1">Yes</option>
</select>
<div id="hidden_div" style="display:none;">Hello hidden content</div>
<script type="text/javascript">
function showDiv(select){
if(select.value==1){
document.getElementById('hidden_div').style.display = "block";
} else{
document.getElementById('hidden_div').style.display = "none";
}
}
</script>
_x000D_